FIG. 1 shows a prior art OFDM transmitter 100. MAC (Medium Access Controller) 102 and associated circuitry, which generate a stream of digital payload data information for forming the payload of a wireless packet. A preamble is pre-pended by function 104 and a packet header including data rate, encoding type, packet length and other information is added by header insertion function 108, and the content is scrambled 112, encoded 116, punctured 118, interleaved 120, and modulated into a plurality of subcarriers 122. An IFFT function 124 converts the complex valued frequency domain modulation information provided by 122 into a complex valued time domain waveform having I and Q components, which is filtered 126, converted to an analog signal 127, and low pass filtered 132 to remove out of band components introduced by the conversion, mixed to a carrier frequency such as 2.4 Ghz by mixer 128 and transmit oscillator 130, and applied to a power amplifier 134, which is coupled to an antenna 136 for transmission through the wireless media.
In wireless systems, such as wireless LANs, there is often interference due to nodes within a network or from other networks. There is also the effect of channel, thermal noise and other RF impairments. In many situations the goal is to maximize the throughput in a link between two nodes. In a WLAN system, for example, it is desirable to maximize the throughput between an AP and a station. In most wireless systems such as WLAN, there is a set of modulation and code rates that the transmitter can select to use in transmitting data to the intended receiver, which are performed by the modulator 122 of FIG. 1 with the associated modulation index provided to header insert function 108. Modulation and coding selection (MCS) is the term used to describe the process of evaluating possible modulations and code rates for use in a particular communications link, and the currently available modulation and code rates for IEEE 802.11n (1 to 4 spatial streams) are shown below for reference:
TABLE 1Data rate (Mbit/s)20 MHz40 MHzchannelchannelModu-800400800400MCSSpatial lationCodensnsnsns indexstreamstyperateGIGIGIGI01BPSK½6.57.213.51511QPSK½1314.4273021QPSK¾19.521.740.5453116-QAM½2628.954604116-QAM¾3943.381905164-QAM⅔5257.81081206164-QAM¾58.565121.51357164-QAM⅚6572.213515082BPSK½1314.4273092QPSK½2628.95460102QPSK¾3943.3819011216-QAM½5257.810812012216-QAM¾7886.716218013264-QAM⅔104115.621624014264-QAM¾117130 24327015264-QAM⅚130144.4 270 300163BPSK½19.521.740.545173QPSK½39 43.38190183QPSK¾58.565121.513519316-QAM½7886.716218020316-QAM¾117130.724327021364-QAM⅔156173.332436022364-QAM¾175.5195364.540523364-QAM⅚195216.7405450244BPSK½2628.85460254QPSK½5257.6108120264QPSK¾7886.8162 18027416-QAM½104115.621624028416-QAM¾156173.232436029464-QAM⅔208231.243248030464-QAM¾234260 48654031464-QAM⅚260288.8 540600
In the prior art, Modulation and Coding Selection (MCS) typically follows a heuristic model, where the repetitive failure of a particular MCS results in the selection of modulation and coding associated with a successively slower data rate until the channel communication is functioning with satisfactory throughput. Satisfactory throughput performance of prior art wireless systems is determined by the receipt of acknowledgement packets for each previously transmitted packet, indicating the channel is not losing packets. Packets which are corrupted during the receive process, such as by multi-path reflection or from an interferer on an adjacent channel or the same channel, or from a device such as a microwave oven operative on the ISM (industrial, scientific, medical) frequency band shared with WLAN networks, may require different treatment depending on the underlying cause for the packet being lost or not being decoded by the recipient.
There are many different causes for loss of throughput when using an MCS which was previously satisfactory. One cause for reduced throughput leading to reduced rate MCS is greater separation or a change in orientation between transmitter and receiver, thereby causing a change in the wireless channel, or interference on the WLAN. Another cause for reduced channel throughput is high channel utilization resulting in increased collisions between the various transmitters sharing the highly utilized channel. Each of these issues requires a different solution. For example, if the underlying cause of loss of throughput is collision or interference from other packets, then a reduction in data rate such as by a new MCS selection will result in increased packet duration and associated consumption of time intervals on the channel. This increased consumption of time by packets on the highly utilized channel results in greater chance of collision and also consumes valuable available time for transmission by other devices, reducing throughput for the other devices on the network.
One prior art MCS selection algorithm is known as the minstrel algorithm and has been used widely. In this heuristic algorithm, the throughput across all the possible MCS in a particular wireless link are obtained by actual throughput measurement for each modulation type and data rate, and saved into a throughput table of entries showing throughput for each data rate, which is updated over time. Additionally, the selection of modulation type and rate may also be modified by a weighting function which gives more weight to newer results than older results. The optimal MCS is then chosen based on examination of the table of MCS vs throughput table. The minstrel algorithm assumes the throughput vs. MCS behavior is unknown and aims to obtain it by actual measurements.
It is desired to provide a method and apparatus for achieving a maximum utilization of the wireless network in the presence of interference sources.